Titanium dioxide films (layers) are known for their semiconductive properties and this property renders them useful for photovoltaic cells. However, titanium dioxide has a large band gap and therefore it does not absorb light in the visible region of the spectrum. For solar applications it is important that the titanium dioxide film be coated with a photosensitiser which harvests light in the wavelength domain where the sun emits light, i.e. between 300 and 2000 nm. Thermodynamic considerations show that conversion of solar energy into electricity is achieved in the most efficient fashion when all the emitted photons with wavelengths below 820 nm are absorbed by the photosensitizer. The optimal dye for solar conversion should therefore have an absorption onset around 800 nm and the absorption spectrum should be such that it covers the whole visible domain.
A second requirement for efficient solar light energy conversion is that the dyestuff after having absorbed light and thereby acquired an energy-rich state is able to inject with practically unit quantum yield, an electron into the conduction band of the titanium dioxide film. This requires that the dyestuff be attached to the surface of the titanium dioxide through suitable interlocking groups. The function of the interlocking group is to provide electronic coupling between the chromophoric group of the dyestuff and the conduction band of the semiconductor. This type of electronic coupling is required to facilitate electron transfer between the excited state of the dyestuff and the conduction band. Suitable interlocking groups are .pi.-conducting substituents such carboxylate groups, cyano groups, phosphate groups or chelating groups with .pi.-conducting character such as oximes, dioximes, hydroxy quinolines, salicylates and alpha keto enolates. The electrons, photoinjected by the dyestuff, generate electrical current in the external circuit when the photovoltaic cell is operated.